Not applicable.
The present invention relates to a method and a system in a paper machine or the like for passing a web from the press section to the drying section. Thus the invention relates particularly to such paper machine solutions, where the web is passed from the press section to the drying section in a closed draw, i.e. directly from the press fabric, felt or other belt, such as a transport belt, to the drying wire of the first drying group of the drying section. The web is passed with the aid of a transport suction roll.
In a drying section provided with a single wire draw, the web is directed in the first drying group, supported by the wire, over the first drying cylinder and from there further in a way known as such, supported by a suction roll or the like, to the second drying cylinder of the drying group, and so on, alternately over a hitch roll, typically a suction roll, and alternately over a drying cylinder. The passage of the web from a drying cylinder to a hitch roll is stabilised in a manner known as such, with the aid of a vacuum box arranged in a pocket formed between the drying cylinders and the hitch roll, preferably at the opening gap between the wire and the first drying cylinder in the travelling direction of the web. Corresponding vacuum boxes are typically arranged also in the pockets formed between the next drying cylinders and the hitch rolls.
FIG. 1 shows a typical travel of the web w in the opening gap between the drying cylinder A and the drying wire F. As shown in FIG. 1, the web tends to follow the surface of the drying cylinder and momentarily to become detached from the drying wire. As the web is detached it creates a pouch, which is difficult to return into contact with the wire in a neat manner. Further problems are caused by the fact that the wire does not detach from the surface of the drying cylinder in a completely symmetrical manner in the cross direction of the web, whereby the travel of the web is not completely uniform even in the cross direction of the web, and pouches, folds and other damages, even breaks, are caused in the web. Pouches in the machine direction are visible all the way to the next cylinder.
The above mentioned problems occur particularly at high running speeds, and when the effect of the vacuum box is not sufficient to control the travel of the web in the opening gap. Within certain limits it is possible to treat the problems with the aid of vacuum boxes, i.e. it is possible to aid the web to detach from the drying cylinder and to be reattached to the drying wire.
However, it may be problematic to adjust the negative pressure in the vacuum box, so that full control over the travel of the web is obtained. If the negative pressure of the vacuum box is too low, e.g. xe2x88x92100 Pa, there will be difficulties in detaching the wire from the cylinder, which results in the above mentioned problems. However, a low negative pressure is advantageous in that the negative pressure returns a detached web to the wire at a small angle, i.e. in a relatively well controlled manner. If, on the contrary, the negative pressure is relatively high, e.g. xe2x88x92500 Pa . . . xe2x88x921000 Pa, depending on the box type, then the web will detach more easily from the cylinder and it will not form a pouch which is as large as when running at a low negative pressure. However, then there may rise a problem in that the web is returned too violently to the wire. A high negative pressure will violently suck the web with a sharp twitch against the wire, which may cause damages in the web or further weaken already weak spots in the web, which easily results in a web break in the respective pocket or later on.
A too high negative pressure also causes difficulties by bending the wire towards the vacuum box. Thus a high negative pressure will hinder the wire from passing straight towards the hitch roll, but it will form an arch on the section between the drying cylinder and the hitch roll. Then the wire may come into contact with structures of the vacuum box, if the box is not shaped to be arched in a corresponding manner. Even the shaping of the box will not always help, as the wire is arched in a different way in different phases of a run.
The detachment of the web can be secured and bending of the wire can be avoided by using a vacuum box divided into different negative pressure levels, such as the HiRun box presented in WO 00/50693 of Metso Paper, Inc., where the negative pressure in the travel direction of the web in the first part of the box, i.e. at the opening gap, is higher than in the next part or parts of the box.
However, it would be advantageous if the detachment of the web from the drying wire could be totally or almost totally avoided, and if the web could be made to pass from the drying cylinder to the hitch roll in a controlled manner, so that it follows the wire.
A poor detachment of the web from the cylinder, and the creation of pouches, folds or other damages makes it more difficult to run the web in the next stages. A web break due to a poor detachment can occur both immediately after the gap and even after several drying cylinders. The number of breaks in the drying section depends also on other interfering factors impeding the travel of the web at the beginning of the drying section. There will be less breaks if the web passes faultlessly in the gaps opening from the cylinders. The number of breaks can on their part be regarded to represent the runability.
Many factors have an effect on the above mentioned travel of the web in the opening gap of a drying cylinder, and thus on the runability in the drying section, such factors being e.g.
the quality of the pulp, e.g. the length of the fibres,
the dry solid contents of the web,
the tension difference between the press section and the drying section,
the mechanical shaping of the web in the press section and in the passing of the web before the drying cylinder,
the relaxation velocity of the web
the strength of the web, and
the temperature of the cylinders. Attempts have been made to act on some of these factors in order to improve the runability.
In order to remove the problems caused by the detachment of the web there has been traditionally attempts e.g. to increase the tension difference between the press section and the drying section. This causes an internal tension in the web, which aids in detaching the web from the cylinder and thus it improves the runability. However, there is a limit to the increase of the tension difference. An increased tension difference increases the number of breaks. Further, a too much increased tension difference has an disadvantageous effect for instance on the quality characteristics of the web. As the speeds of the paper machine increase there has also emerged a growing need to increase the tension differences in order to retain the runability.
There have also been attempts to keep the temperature of the surface of the drying cylinder at the beginning of the drying section on a level at which the web does not bum to the surface of the cylinder. This solution is less good regarding the drying efficiency of the drying section.
The above mentioned runability problems at the beginning of the drying section have among other things often resulted in that it has been necessary to shorten the length of the cylinder group at the beginning of the drying section, which increases costs.
The object of the invention is to achieve a method and a system for passing a paper web or the like from the press section to the cylinder drying group, where the above mentioned problems are minimised.
Thus an object of the invention is to provide a reliable method and system for passing a paper web or the like from a press section to a cylinder drying group.
Then the object of the solution according to the invention is among other things to pass the web from the press section to the drying section with a minimum tension difference, in some cases even completely without tension, or even with a negative tension difference, however, maintaining the optimal running conditions in the drying section and a good paper quality.
A further object is to provide a method and a system for passing the web from the press section to the drying section so that a good runability is maintained in the drying section also with drying groups of traditional lengths, i.e. with unshortened drying groups.
Thus the solution according to the invention proposes to use a substantially higher negative pressure level than conventionally in the transfer suction roll of the drying section. The transfer suction roll should typically maintain a negative pressure which is between xe2x88x9235 kPa and xe2x88x9280 kPa, typically between xe2x88x9240 kPa and xe2x88x9265 kPa, most typically  greater than xe2x88x9250 kPa.
Previously it was not considered necessary to maintain a high negative pressure level in this transfer suction roll. The object has been only to detach the web from the fabric of the press section or belt and to pass it to the wire of the drying section. The negative pressure of the transfer suction roll was not considered to have any effect after this passing, and thus no effect on the run of the web, on the runability, or on the drying process in the drying groups. Often it is secured that the web is kept on the drying wire immediately after the passing, for instance with the aid of vacuum boxes.
However, now it was found that the transfer suction roll can be used, not only to pass the web away from the felt of the press section, but also to attach the web to the wire of the drying section in a more robust manner than previously, so that this attachment has an effect also after the opening gap of the first drying cylinder. It was found that when the web is attached better than previously to the wire at the transfer suction roll, this will have an effect even at the opening gap after the drying cylinder, where a web which is once attached to the wire in a proper manner will detach from the drying cylinder in a more easier manner than previously. Previously the detachment of the web from the cylinder was aided by increasing the tension difference, i.e. by increasing the tension in the web. The tension difference in the web is increased as it passes from one fabric to another, i.e. at the suction roll.
Previously an aim was to keep the web attached to the wire of the drying section with the negative pressure level of the vacuum box in the pocket between the first and second drying cylinders, or to attach the web again to this wire if it detached. However, a disadvantageous negative pressure level could cause an unstable travel of the web in the opening gap, whereby any detachment of the web may cause pouches or folds in the web, which impairs the runability and the paper quality.
As described above, an aim has been to compensate for the problems caused by pouches and folds in the web by increasing the tension difference between the press section and the drying section. Then it was often necessary to increase the tension difference even up to the tolerance limits of the web.
In the solution according to the invention it is therefore an aim to minimise the detachment of the web from the drying wire by attaching it particularly well to the wire before the drying cylinder. During the whole pass in the opening gap the web is kept in this way on the surface of the wire, or is detached from it only in an unsubstantial way, i.e. so little that the passage of the web is fully controlled, both in the opening gap and after the gap.
In the solution according to the invention the aim is to secure that the attachment between the web and the drying wire which occurs at the transfer suction roll will last as long as possible. As a result it is not necessary to make the negative pressure of the vacuum box after the cylinder as high as in the known solutions in order to control the runability.
In the solution according to the invention the negative pressure prevailing in the transfer suction roll and the tension difference between e.g. the press section and drying section can be adjusted to such a level that both an optimal running situation and optimal paper quality criteria are obtained. The adjustment takes into account the general running situation, the equipment of the paper machine, the pulp quality and characteristics, as well as the quality of the paper being manufactured.
According to the invention the high negative pressure of the transfer suction roll has a particularly favourable effect on the runability at high running speeds, for instance over 1200-1500 m/min, depending on the paper quality. At low running speeds, e.g. below 1000-1200 m/min, the negative pressure level has not so substantial an effect, but a slightly lower negative pressure than normal will be sufficient.
The higher negative pressure in the transfer suction roll, the better the web will tend to follow the wire. As the negative pressure increases the contact surface between the web and the wire will increase. The dry solid contents of the web also has an influence. The wetter the web, the more easily the web will be shaped according to the surface of the wire and increase the attachment area. A risk with a too high negative pressure can be that the web is sucked into the wire, whereby the wire may cause a marking in the web, which is not desired.
Previously it was necessary to choose the tension difference between the press section and the drying section in accordance with the runability. According to the invention, the runability can now be controlled also through the negative pressure of the transfer suction roll, and thus the tension difference will not have an equally great importance, if any, regarding the runability. When using the solution according to the invention the tension differences can be reduced to a substantially lower level, compared to those used previously.
The selection of the tension difference is normally influenced by the paper quality and the machine speed. As the negative pressure is kept relatively high according to the invention, then the tension difference can be typically adjusted as follows:
in the region  less than 1%, when the machine speed is 600 to 1200 m/min;
in the region  less than 1.5%, when the machine speed is 1200 to 1500 m/min;
in the region  less than 2%, when the machine speed is 1500 to 1800 m/min;
in the region  less than 2.5%, when the machine speed is  greater than 1800 m/min.
The machine can be run even with a tension difference of zero by utilising the solution according to the invention. In test runs it has been found that even at a speed of 1600 m/min it is possible to run with a tension difference of zero. In a conventional production machine, i.e. with a lower negative pressure in the transfer suction roll, the tension difference should be of the order of 3 to 4% at a speed of 1600 m/min.
The solution according to the invention, where the selection of the tension difference does not have so great effect on the runability, the tension difference can be chosen according to other criteria than the runability. The tension difference can be chosen for instance according to requirements put by the paper quality to be manufactured or by the paper characteristics, such as porosity, stretch at break, strength, tensile energy absorption, dimensional stability, or by other properties.
The refining degree can also have an effect on the porosity, whereby it is possible with e.g. a low tension difference (0.5%) and a low refining degree (100 kWh) to save energy and maintain the same porosity as with a high tension difference (2%) and a high refining degree (150 kWh/tn).
In some cases the solution according to the invention also makes it possible to increase the steam pressure of the drying cylinders at the beginning of the drying section, which gives more capacity to the drying section. When the web is well attached to the wire it reduces the risk of a so called web burning to the drying cylinder. Previously it was possible to use only relatively low temperatures in the drying cylinders at the beginning of the drying section in order to avoid burning.